1. Field of the Invention
This invention relates generally to a torque converter for an automatic transmission, and, in particular, to a hydraulic system that actuates an impeller clutch of the torque converter and provides a continuous supply of hydraulic lubricant to transmission components.
2. Description of the Prior Art
A torque converter is a modified form of a hydrodynamic fluid coupling, and like a fluid coupling, is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. A torque converter is able to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear.
In a torque converter there are at least three rotating elements: an impeller, which is mechanically driven by the prime mover; a turbine, which drives the load; and a stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller to multiply torque. The stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating the prime mover but allows for forward rotation. The torque converter is encased in a housing, which contains with automatic transmission fluid (ATF), sometimes referred to as “oil,” “lube” or “lubricant.”
Hydrodynamic parasitic losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a bypass clutch (also called lock-up clutch), which physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and therefore virtually no power loss and improved fuel economy.
Torque converter clutch designs include two basic types, a closed piston design and an open piston design. A closed piston design requires a dedicated hydraulic circuit into the torque converter, which communicates only with the apply side of the clutch piston. When pressure is high, the clutch applies. When pressure is low, the clutch releases. A more uncommon form is to have this circuit on the release side where high pressure releases the clutch and low pressure applies the clutch.
An open piston design involves flowing ATF through the torque converter and across the piston, flowing from the apply side to the release side. The piston is applied by the pressure difference between the apply and release sides. This pressure differential can be controlled by either controlling apply and release pressure directly or by controlling flow rate with a designed pressure drop restriction across the piston. Normally, this same flow of ATF is used to cool the torque converter, so a relatively high flow rate is required in this hydraulic circuit. A barrier to achieving the intended flow rate is the limitation on converter charge pressure to prevent converter ballooning (axial distortion of the torque converter). This commonly results in a high gain clutch design where small pressure drop changes across the piston result in large changes in apply force, which makes clutch controllability a challenge.
Most torque converters only have one converter clutch, the bypass clutch which alternately connects and releases a drive connection between the impeller and turbine. A torque converter can also provide an impeller clutch to connect and release a drive connection between the impeller and a power source, such as an engine, electric motor, starter/generator or hydraulic motor. The intent of the impeller clutch is to reduce load on the power source during idle, which reduces fuel consumption. This functionality is commonly referred to as idle-disconnect or neutral idle.
When two clutches are present within a torque converter, usually one piston is an open piston design while the other is a closed piston design. Having two closed piston designs within a torque converter is not practical because this requires four hydraulic circuits to communicate with the torque converter—one for each clutch, and two more to flow across the converter for cooling. Having two as open piston clutches presents a complicated design problem for controlling the apply and release of the two clutches independently. This leads to the more practical approach of using a closed piston design for the bypass clutch and an open piston design for the impeller clutch.
In an open piston design, the impeller clutch is actuated by a pressure differential between a converter charge circuit and a converter discharge circuit. A relatively high flow rate is required to cool the converter when the impeller clutch is engaged. Low flow restrictions across the closed clutch to reduce the pressure drop and a high gain clutch to maintain capacity would be required to avoid an excessive charge pressure. To disengage the impeller clutch, the pressure drop must be reduced lower yet. Because there is no direct control over flow restrictions across the clutch, pressure drop can only be reduced by reducing flow rate through the converter. During vehicle launch, the ramp rate of pressure drop across the converter clutch can be varied to achieve a variable “k factor” across the clutch for better launch feel.
When the torque converter is multiplying torque, power loss occurs which significantly increases the temperature of ATF in the torque converter and must be cooled before returning to the transmission. Cooler return oil is usually routed into the transmission lubrication circuit to cool internal clutches, gear sets and bearings. The lubrication circuit is also used to fill or charge balance dams, which are intended to keep disengaged clutch pistons from drifting on when internal rotational speeds increase.
The converter clutch control and hydraulic layout described above reduces flow to the downstream lube circuit when in idle-disconnect mode. When in idle-disconnect mode, the balance dams will drain down and result in an error state during the upcoming drive-away unless a minimum lubrication circuit flow rate is maintained, which cannot easily be met and still have the low pressure drop needed to disengage the impeller clutch. This error state could cause unintended clutch application including an early gear shift, an unintended gear state, or a tie up in the gearbox.
There is a need in the industry to control an impeller clutch in a torque converter during idle-disconnect mode without introducing risk to the transmission lubrication system.